[NDF] Implement TODO about thread safety in churn limiter

MERGE REQUEST

MR Guidelines

Overview

This MR implements a todo in the churn limiter which mentioned a potential race condition. This seems to at least fix the following NDFS.

 --- FAIL: TestSelectRandomCanceledHost (6.58s)
     hostdb_test.go:389: Expected 1 inactive contract but got 0
 --- FAIL: TestContractorChurnLimiter (44.93s)
     contractor_test.go:1846: expected 1 disabled contracts but got 2
 --- FAIL: TestRenterFileContractIdentifier (6.17s)
     renter_test.go:3243: Expected 4 txns but got 2

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods or updated methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to workflow
• Issue added to Sia-UI repo for new supporting features
• Changelog File Created